The target polynitro compound, ε-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (hereinafter only ε-CL20) belongs to the group of polycyclic nitramines and finds applications predominantly in military high explosive mixtures and propellants (Song Zhenwei; Li Xiaojiang: Recent research progress and application prospect of high energy density compound HNIW, Huaxue Tuijinji Yu Gaofenzi Cailiao (2011), 9(1), 40-45; U. R. Nair, R. Sivabalan, G. M. Gore, M. Geetha, S. N. Asthana, H. Singh: Hexanitrohexaazaisowurtzitane (CL-20) and CL-20-based formulations, Combustion, Explosion, and Shock Waves, (2005) 41(2) 121-132). This high explosive belongs among those with the highest performance, but its wider use is hindered by relatively high manufacturing costs and particularly by its unsatisfactory sensitivity—there still prevails the view that the sensitivity of CL20 is approximately at the level of that of pentaerythritol tetranitrate (PETN), i.e. max. 4 J in the impact sensitivity (U. R. Nair, R. Sivabalan, G. M. Gore, M. Geetha, S. N. Asthana, H. Singh: Hexanitrohexaazaisowurtzitane (CL-20) and CL-20-based formulations, Combustion, Explosion, and Shock Waves, (2005) 41(2) 121-132). The sensitivity, particularly the impact sensitivity, of this type of compounds is affected by the purity of the nitramine as well as by the size, break-down rate and shape of its crystals. A perfect removal of side products from the crystals of CL20 is not easy and, therefore, the standard NATO STANAG-4566 allows their maximum content in the final product to be 3% w/w. The break-down rate of the crystals, i.e. their porosity and cracks, is connected with the recrystallization method and the solvents chosen for the recrystallization; as far as the shape of the crystals is concerned, desirable are spherical particles or, more precisely, crystals with rounded edges. The literature comprises a large number of articles about the recrystallization of CL20 (e.g., N. Degirmenbasi, Z. Penalta-Inga, U. Olgun, H. Gocmez, D. M. Karyon: Recrystallization of CL20 and HNFX from solution for rigorous control of the polymorph type: Part II, experimental studies, Journal of Energetic Materials (2006) 24, 103-139).
The recrystallization of CL20, which is focused on the preparation of its technologically most attractive ε-modification, mostly involves the application of a “solvent-antisolvent” system (the product is precipitated from its solution by the addition of another solvent in which it is poorly soluble or completely insoluble). The solvents used in this method often include ethyl acetate or its mixture with toluene, but there are other solvents patented, e.g., methyl acetate, isopropyl acetate, butyl acetate, tetrahydrofurane, methyl ethyl ketone, cyclohexanone, acetonitrile and acetone. When α-CL (which, being a hemihydrate, contains crystal water) is the starting modification used for the recrystallization, water must be removed from the solution of α-CL either by addition of a drying agent, such as magnesium or sodium sulfate, potassium carbonate or silica gel, or by azeotropic distillation with ethyl acetate, or ethyl acetate and toluene. The thus dried and optionally subsequently filtered CL20 solution is concentrated for recrystallization (when the ethyl acetate plus toluene mixture is used) or precipitated in a predefined way by addition of hexane, heptane, cyclohexane, octane, petroleum ether, dichloromethane, trichloromethane, benzyl acetate, benzyl formate, toluene, xylene or even benzene (WO 99/57104; U.S. Pat. Nos. 5,973,149; 5,874,574; US 2003/0130503; US 2003/636373; JP 11322752; EP 0913374). However, this method does not necessarily lead to a substantial reduction of the content of impurities in the product. The laboratory methods of preparation of highly pure CL20 include filtration with charcoal, e.g., in the amount of 20 g charcoal per 1 g CL20 (A. J. Bellamy: A simple method for the purification of crude hexanitrohexaazaisowurtzitane, Propellants, Explosive, Pyrotechnics, (2003) 28(3) 145-152) or column chromatography of benzene solution of CL20 (Ou Yuxiang et al.: Synthesis and crystal structure of β-hexanitrohexaazaisowurtzitane, Science in China, Series B, (1999) 42(2) 218-224).
All the above described recrystallization methods destined for the preparation of ε-CL20 yield a product having the impact sensitivity of about 4 J, which restricts its application as an active component particularly in special military ammunition.
Only four publications deal with the preparation of the very attractive ε-CL20 with spherical crystals (i.e. crystals with rounded edges exhibiting the reduced impact sensitivity, RS-CL20):    a) in the application of the “solvent-antisolvent” system, the solution of CL20 in ethyl acetate was precipitated by addition of heptane fraction with a simultaneous ultrasound treatment (R. Sivabalan, G. M. Gore, U. R. Nair, A. Saikia, S. Venugopalan, B. R. Gaandhe: Study on ultrasound-assisted precipitation of CL20 and its effect on morphology and sensitivity, Journal of Hazardous Materials (2007) A139, 199-203); this sonication shortened the precipitation time and yielded RS-CL20 crystals exhibiting impact sensitivities of 5.9 J to 10.8 J. However, the description of the procedure does not mention how the procedure affected the product purity.    b) according to the Chinese patent document CN 101624394 (2010), a recrystallization promoter is added into the solution before precipitation; the promoter may include organic compounds of the amino acid group, such as glycine, alanine etc., and esters of these amino acids, polyols, such as poly(vinyl alcohol), glycerol, pentaerythritol and others, organic acids, such as butanecarboxylic, maleic, malonic, adipic acid etc., and esters of these acids. The RS-CL20 obtained in this way has the purity above 98% and the impact sensitivity of 5.6 to 10.2 J. A similar procedure is protected by a Korean patent KR 224043 (1999) describing the use of pyridine as the promoter and yielding RS-CL20 with the impact sensitivity of 8.1 J.    c) Chinese authors also used a precipitation recrystallization from supercritical solution of CL20 in liquid carbon dioxide (gas-antisolvent; Hu Li-shuang, Hu Shuang-qi: Study on the application of the supercritical solution technology in preparation of Ultra-fine and sphere CL-20; Zhongguo Anquan Shengchan Kexue Jishu (2010), 6(3), 80-83). This method is demanding with respect to the necessary technological equipment (high pressures).
The aim of the present invention is, while taking into account the above-mentioned findings about the so far known procedures of preparation of RS-CL20, to provide a new procedure, which—without high demands for the equipment and materials—would yield the required spherical crystals of the product, while at the same time substantially decreasing the amount of impurities in the final product and ensuring reproducibility in achieving the reduced impact sensitivity.